Devices for integrated, repeated, prolonged, and/or reliable sweat stimulation and biosensing

ABSTRACT

A sweat sensing device includes a plurality of sweat collection pads communicating with a sensor. Each of the pads is activated by a timing circuit which allows one or more of the pads to be activated at a selected time and subsequent deactivated after a defined period of time. This allows for selective collection of sweat from a plurality of pads over a prolonged period of time. An impedance measuring circuit can be employed to determine if one or more of the pads becomes disconnected, in order to avoid irritation. Further, the devices can use a common microfluidic device which both transports sweat activating substance, such as pilocarpine, to the surface of the skin and directs sweat away from the skin to a sensing device.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Applications No.61/892,859, entitled “SWEAT STIMULATION FOR INTEGRATED OR REPEATEDBIOSENSING” filed Oct. 18, 2013, and 62/003,707 entitled “DEVICECONSTRUCTION FOR PROLONGED AND RELIABLE SWEAT STIMULATION AND SENSING”filed May 28, 2014, the disclosures of which are hereby incorporated byreference herein in their entirety. The disclosure of PCT/US13/35092,filed Apr. 3, 2013 is also incorporated herein by reference in itsentirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

The present invention was made, at least in part, with support from theU.S. Government and funds identified as SAPGrant No. 1008512, awarded bythe U.S. Air Force Research Labs. The U.S. Government has certain rightsin the present invention.

BACKGROUND OF THE INVENTION

Sweat sensing technologies have enormous potential for applicationsranging from athletics, to neonates, to pharmacological monitoring, topersonal digital health, to name a few applications. This is becausesweat contains many of the same biomarkers, chemicals, or solutes thatare carried in blood, which can provide significant information whichenables one to diagnose ailments, health status, toxins, performance,and other physiological attributes even in advance of any physical sign.Furthermore sweat itself, and the action of sweating, or otherparameters, attributes, solutes, or features on or near skin or beneaththe skin, can be measured to further reveal physiological information.

Sweat has significant potential as a sensing paradigm, but it has notemerged beyond decades-old usage in infant chloride assays for CysticFibrosis (e.g. Wescor Macroduct system) or in illicit drug monitoringpatches (e.g. PharmCheck drugs of abuse patch by PharmChem). Themajority of medical literature discloses slow and inconvenient sweatstimulation and collection, transport of the sample to a lab, and thenanalysis of the sample by a bench-top machine and a trained expert. Allof this is so labor intensive, complicated, and costly, that in mostcases, one would just as well implement a blood draw since it is thegold standard for most forms of high performance biomarker sensing.Hence, sweat sensing has not achieved its fullest potential forbiosensing, especially for continuous or repeated biosensing ormonitoring. Furthermore, attempts at using sweat to sense ‘holy grails’such as glucose have failed to produce viable commercial products,reducing the publically perceived capability and opportunity space forsweat sensing. A similar conclusion has been made very recently in asubstantial 2014 review provided by Castro titled “Sweat: A sample withlimited present applications and promising future in metabolomics”,which states: “The main limitations of sweat as clinical sample are thedifficulty to produce enough sweat for analysis, sample evaporation,lack of appropriate sampling devices, need for a trained staff, anderrors in the results owing to the presence of pilocarpine. In dealingwith quantitative measurements, the main drawback is normalization ofthe sampled volume.”

Many of these drawbacks stated above can be resolved by creating noveland advanced interplays of chemicals, materials, sensors, electronics,microfluidics, algorithms, computing, software, systems, and otherfeatures or designs, in a manner that affordably, effectively,conveniently, intelligently, or reliably brings sweat sensing technologyinto intimate proximity with sweat as it is generated. Sweat sensingtherefore becomes a compelling new paradigm that clearly was overlookedin terms of its ultimate potential as a biosensing platform.

Sweat sensors have many potential advantages over other biofluidsensors. But one potentially confounding factor is that prolongedstimulation of sweat can be problematic as some individuals can be hypersensitive to prolonged stimulation of sweat or their glands will adaptto sweat stimulation and provide no or reduced response to sweatstimulation by heat, electricity, iontophoresis, or other means.Furthermore, for prolonged stimulation, risk of electrode detachment isa risk, and can even be a risk at the onset of stimulation. Solutionsfor solving these risks are lacking.

The number of active sweat glands varies greatly among different people,though comparisons between different areas (ex. axillae versus groin)show the same directional changes (certain areas always have more activesweat glands while others always have fewer). The palm is estimated tohave around 370 sweat glands per cm²; the back of the hand 200 per cm²;the forehead 175 per cm²; the breast, abdomen, and forearm 155 per cm²;and the back and legs 60-80 per cm². Assuming use of a sweat glanddensity of 100/cm², a sensor that is 0.55 cm in radius (1.1 cm indiameter) would cover ˜1 cm² area or approximately 100 sweat glands.According to “Dermatology: an illustrated color text” 5th edition, thehuman body excretes a minimum of 0.5 liter per day of sweat, and has 2.5million sweat glands on average and there are 1440 minutes per day. Forprepubescent children, these sweat volumes are typically lower. For 2.5million glands that rate is 0.2 μl per gland per day or 0.14nl/min/gland. This is the minimum ‘average’ sweat rate generated perpore, on average, with some possible exceptions being where sweatingincreases slightly on its own (such as measuring sleep cycles, etc.).Again, from “Dermatology: an illustrated color text” 5th edition, themaximum sweat generated per person per day is 10 liters which on averageis 4 μL per gland maximum per day, or about 3 nL/min/gland. This isabout 20× higher than the minimum rate.

According to Buono 1992, J. Derm. Sci. 4, 33-37, “Cholinergicsensitivity of the eccrine sweat gland in trained and untrained men”,the maximum sweat rates generated by pilocarpine stimulation are about 4nL/min/gland for untrained men and 8 nL/min/gland for trained(exercising often) men. Other sources indicate maximum sweat rates of anadult can be up to 2-4 liters per hour or 10-14 liters per day (10-15g/min·m²), which based on the per hour number translates to 20nL/min/gland or 3 nL/min/gland. Sweat stimulation data from“Pharmacologic responsiveness of isolated single eccrine sweat glands”by K. Sato and F. Sato (the data was for extracted and isolated monkeysweat glands, which are very similar to human ones), suggests a rate upto ˜5 nL/min/gland is possible with stimulation, and several types ofsweat stimulating substances are disclosed. For simplicity, we canconclude that the minimum sweat on average is ˜0.1 nL/min/gland and themaximum is ˜5 nL/min/gland, which is about a 50× difference between thetwo.

Based on the assumption of a sweat gland density of 100/cm², a sensorthat is 0.55 cm in radius (1.1 cm in diameter) would cover ˜1 cm² areaor approximately 100 sweat glands. Assuming a dead volume under eachsensor of 50 μm height or 50×10⁻⁴ cm, and that same 1 cm² area, providesa volume of 50E-4 cm³ or about 50E-4 mL or 5 μL of volume. With themaximum rate of 5 nL/min/gland and 100 glands it would require 10minutes to fully refresh the dead volume. With the minimum rate of 0.1nL/min/gland and 100 glands it would require 500 minutes or 8 hours tofully refresh the dead volume. If the dead volume could be reduced by10× to 5 μm roughly, the max and min times would be 1 minute and 1 hour,roughly respectively, but the min rate would be subject to diffusion andother contamination issues (and 5 μm dead volume height could betechnically challenging). Consider the fluidic component between asensor and the skin to be a 25 μm thick piece of paper or glass fiberwith, which at 1 cm² equates to a volume of 2.5 μL of volume and if thepaper was 50% porous (50% solids) then the dead volume would be 1.25 μL.With the maximum rate of 5 nL/min/gland and 100 glands it would require2.5 minutes to fully refresh the dead volume. With the minimum rate of0.1 nL/min/gland and 100 glands it would require ˜100 minutes or ˜2hours to fully refresh the dead volume.

Sweat stimulation is commonly known to be achieved by one of severalmeans. Sweat activation has been promoted by simple thermal stimulation,by intradermal injection of drugs such as methylcholine or pilocarpine,and by dermal introduction of such drugs using iontophoresis. Gibson andCooke's device for iontophoresis, one of the most employed devices,provides DC current and uses large lead electrodes lined with porousmaterial. The positive pole is dampened with 2% pilocarpinehydrochloride, and the negative one with 0.9% NaCl solution. Sweat canalso be generated by orally administering a drug. Sweat can also becontrolled or created by asking the subject using the patch to enact orincrease activities or conditions which cause them to sweat.

Sweat rate can also be measured real time in several ways. Sodium can beutilized to measure sweat rate real time (higher sweat rate, higherconcentration), as it is excreted by the sweat gland during sweating.Chloride can be utilized to measure sweat rate (higher sweat rate,higher concentration), as it is excreted by the sweat gland duringsweating. Both sodium and chloride can be measured using ion-selectiveelectrodes or sealed reference electrodes, for example placed in thesweat sensor itself and measured real time as sweat emerges onto theskin. Sato 1989, pg. 551 provides details on sweat rate vs.concentration of sodium & chloride. Electrical impedance can also beutilized to measure sweat rate. Grimnes 2011 and Tronstad 2013demonstrate impedance and sweat rate correlations. Impedance and Naconcentration, and or other measurements can be made and used tocalculate at least roughly the sweat pore density and sweat flow ratefrom individual sweat glands, and coupled with sweat sensing orcollection area to determine an overall sweat flow rate to a sensor.More indirect measurements of sweat rate are also possible throughcommon electronic/optical/chemical measurements, including those such aspulse, pulse-oxygenation, respiration, heart rate variability, activitylevel, and 3-axis accelerometry, or other common readings published byFitbit, Nike Fuel, Zephyr Technology, and others in the currentwearables space, or demonstrated previously in the prior art.

With reference to FIG. 1A, a prior art sweat stimulation and sensingdevice 10 is positioned on skin 12 and is provided with features shownrelevant to the present invention. The device 10 is adhered to the skin12 with an adhesive 14 which carries a substrate 13, control electronics16, at least one sensor 18, a microfluidic component 20, a reservoir orgel with pilocarpine referred to as pilocarpine source 22, aniontophoresis electrode 24, and counter electrode 26. The electrodes 24and 26 are electrically conductive with and through the skin 12 byvirtue of the conductance of materials 22, 20 and 14 and, in some casesadhesive 14 can be locally removed beneath one or more electrodes orsensors to improve conductance with the skin and/or to improvecollection or interface with sweat. Adhesives can be functional as tackyhydrogels as well which promote robust electrical, fluidic, andiontophoretic contact with skin (as commercially available examples suchas those by SkinTact for ECG electrodes). With reference to FIG. 1B, atop view of connections to the electronics 16 is shown, such connectionsby example only and not representing a limiting configuration. Theelectronics 16 can be a simple as a controlled current source andsensing electronics only, or more complex including computing,communication, a battery, or other features. Again, in some embodiments,the electronics may be much simpler or not needed at all.

With further reference to FIGS. 1A and 1B, if the device 10 were tostimulate sweat by virtue of iontophoretically driving a stimulatingdrug such as pilocarpine from the source 22 into the skin 12, it couldconventionally do so for several minutes and stimulate sweat that couldbe collected for 10-30 minutes by the microfluidic component 20 and flowover the sensor 18 which could detect one or more solutes of interest inthe sweat. This conventional stimulation and collection time frame istypical and similar to that broadly used for infant-chloride assays forcystic fibrosis testing, such as found in products by WescorCorporation. Sweat sensors have advantages over other biofluid sensors,but one potentially confounding factor is that prolonged stimulation ofsweat for more than 30 minutes could be problematic as some individualscan be hyper sensitive to prolonged stimulation of sweat or theirglands, or will adapt to sweat stimulation and provide no or reducedresponse to sweat stimulation by heat, electricity, iontophoresis, orother means. Furthermore, for prolonged stimulation, electrodedetachment can be a risk, or even be a risk at the onset of stimulation.Solutions for solving these risks are lacking. Furthermore, thestimulation can interfere with the quality of the sensing, and thereforeneeds to be resolved as well.

SUMMARY OF THE INVENTION

The present invention is premised on the realization that sweat can beeffectively simulated and analyzed in a single, continuous, or repeatedmanner inside the same device. The present invention addresses theconfounding factors that result in performance being too poor for manypractical uses. Specifically, the present invention provides: sweatsampling and stimulation with at least one shared microfluidiccomponent; sweat sampling and stimulation with at least one component ormembrane added to mitigate the interference of a sweat stimulatingportion of device with the purity of sweat delivery to the samplingportion of the device; multiple stimulation pads and some with their ownsensors; timed pulsing of stimulation in some cases allowing areas ofskin to rest; detection of a faulty stimulation contact with skin; andparametric specification of pads small enough to reduce irritationduring sweat stimulation; and additional alternate embodiments as willbe taught in the specifications.

Further, minimizing dead volume, that is the volume of sweat that mustbe generated to be detected by an electrode or other type of sensor, canin some cases ease some of the challenges of sweat stimulation. Forexample, consider a polymer matrix that is porous to sweat with 10% openporosity, and which is tacky and gel like (so it adheres and bonds toskin). If this were 50 μm thick, then the equivalent dead volume wouldbe that of a 5 μm thick dead volume, the max and min times would be 1minute and 1 hour, roughly respectively, and this is much lesstechnically challenging than a highly open/porous dead volume. Reducingdead volume, isolating sweat pores, minimizing irritation, and otheraspects are all desirable for prolonged stimulation of sweat forchronological monitoring applications. If dead volumes are reducedenough, hourly or even once a day readings are highly possible withoutneed for high sweat rates.

BRIEF DESCRIPTION OF THE DRAWINGS

The objects and advantages of the present invention will be furtherappreciated in light of the following detailed descriptions and drawingsin which:

FIGS. 1A and 1B are side view and top-view diagrams of prior art.

FIG. 2A is a side view diagram sweat sensor device with multiple sweatstimulation pads.

FIG. 2B is an overhead view of FIG. 2A, with only circuitry andelectrodes shown.

FIGS. 3-7 show one of an alternate arrangement for a plurality ofarrangements for sweat stimulation, sweat collection, and sensors.

FIG. 8 shows an embodiment of the present invention that is able todetect an unreliable contact of a sweat stimulation pad with the skin.

FIGS. 9A and 10A are block diagrams of the functionality of integratedsweat stimulation and sweat sampling.

FIGS. 9B and 10B are cross-sectional representations of the embodimentsshown in FIGS. 9A and 10A.

FIG. 11 is a diagrammatic top plan view of the layers used in the deviceshown in FIG. 10A.

DETAILED DESCRIPTION OF THE INVENTION

The detailed description of the present invention will be primarily be,but not entirely be, limited to subcomponents, subsystems, sub methods,of wearable sensing devices, including devices dedicated to sweatsensing. Therefore, although not described in detail here, otheressential features which are readily interpreted from or incorporatedwith the present invention shall be included as part of the presentinvention. The specification for the present invention will providesspecific examples to portray inventive steps, but which will notnecessarily cover all possible embodiments commonly known to thoseskilled in the art. For example, the specific invention will notnecessarily include all obvious features needed for operation, examplesbeing a battery or power source which is required to power electronics,or for example, an wax paper backing that is removed prior to applyingan adhesive patch, or for example, a particular antenna design, thatallows wireless communication with a particular external computing andinformation display device. Several specific, but non-limiting, examplescan be provided as follows. The invention includes reference toPCT/US2013/035092, the disclosure of which is included herein byreference. The present invention applies to any type of sweat sensordevice. The present invention applies to sweat sensing devices which cantake on forms including patches, bands, straps, portions of clothing,wearables, or any mechanism suitable to affordably, conveniently,effectively, intelligently, or reliably bring sweat stimulating, sweatcollecting, and/or sweat sensing technology into intimate proximity withsweat as it is generated. In some embodiments of the present inventionthe device will require adhesives to the skin, but devices could also beheld by other mechanisms that hold the device secure against the skinsuch as strap or embedding in a helmet. The present invention maybenefit from chemicals, materials, sensors, electronics, microfluidics,algorithms, computing, software, systems, and other features or designs,as commonly known to those skilled in the art of electronics,biosensors, patches, diagnostics, clinical tools, wearable sensors,computing, and product design. The present invention applies to any typeof device that measures sweat or sweat rate, its solutes, solutes thattransfer into sweat from skin, a property of or things on the surface ofskin, or measures properties or things beneath the skin.

The present invention includes all direct or indirect mechanisms orcombinations of sweat stimulation, including but not limited to sweatstimulation by heat, pressure, electricity, iontophoresis or diffusionof chemical sweat stimulants, orally or injected drug that stimulatesweat, stimuli external to the body, natural bioactivity, cognitiveactivity, or physical activity. Any suitable technique for measuringsweat rate should be included in the present invention where measurementof sweat rate is mentioned for an embodiment of the present invention.The present invention may include all known variations of biosensors,and the description herein shows sensors as simple individual elements.It is understood that many sensors require two or more electrodes,reference electrodes, or additional supporting technology or featureswhich are not captured in the description herein. Sensors are preferablyelectrical in nature such as ion-selective, potentiometric,amperometric, and impedance (faradaic and non-faradaic), but may alsoinclude optical, chemical, mechanical, or other known biosensingmechanisms. Sensors can allow for continuous monitoring of multiplephysiological conditions realizing larger arrays of biomarker-specificsensors. The larger arrays can determine physiological condition throughsemi-specific but distinct sensors by statistical determination,eliminating the need to quantify individual biomarker levels. Sensorscan be in duplicate, triplicate, or more, to provide improved data andreadings. Many of these auxiliary features of the device may, or maynot, also require aspects of the present invention.

With reference to FIG. 2A, one embodiment of the present invention isdesigned for prolonged and reliable sweat stimulation and sensing.Arrayed stimulation pads can provide the same net effect as one pad forprolonged stimulation (e.g. one long pad for 12 hours of stimulation canbe replaced by an array of 24 pads on the same device of 30 minutesstimulation each). As shown in FIG. 2A, a sweat sensor 28 positioned onskin 12 by an adhesive layer 30 bonded to fluid impermeable substrate32. Substrate 32 holds electronics 34, one or more sensors 36 (oneshown), a microfluidic component 38, coupled to multiple sweat pads 40,42 and 44. The microfluidic component 38 can continuously pump sweat byevaporating sweat (water) from its exposed surface above sensor 36, orcan include an additional continuous pumping mechanism (not shown) suchas addition of a dry hydrogel capable of absorbing or wicking sweat foran extended period of time. As sweat generates its own pressure,microfluidic component 38 could also be a simple polymer microchannel,at least partially enclosed, which is pressure driven. Each pad has asource of sweat stimulant such as pilocarpine 46, 48, 50, andindependently controlled iontophoresis electrodes 52, 54, 56. There isalso one or more counter electrodes 58. To minimize dead volume, thesepads 40, 42 and 44 are preferably less than 1 cm², for example, lessthan 5 mm² down to about 1 mm².

The electronics 34 further include a timing circuit connected to eachelectrode 52, 54, 56 via lines 66, 68 and 70 to promote sweat whendesired. Thus, in operation, the electronics 34 would activate one ofelectrodes 52, 54 or 56 for a defined period of time. This will causegeneration of sweat, which will be transferred through the microfluidicstructure 38, directed to the sensor 36. After a defined period of time,the electronics 34 will discontinue current to electrode 56 and directit to electrode 54, again causing sweat generation beneath electrode 54,but not beneath electrode 56. Again, after a period of time, theelectronics 34 will discontinue current to electrode 54 and beginpassing current to electrode 52, again starting sweat generation beneathelectrode 52 and discontinuing sweat generation beneath electrode 54.Each one of these will direct the sweat through the common microfluidiccomponent 38 to the sensor 36, thus providing long-term generation ofsweat without stressing any particular location on the skin 12 of theindividual.

The sweat pad 60 shown in FIG. 3 represents the case where each pad 60would have its own sensor 62 and microfluidic component 64, along withthe electrode 61 and pilocarpine source 63. A plurality of these wouldbe connected to a common circuit which would activate each pad accordingto a selected schedule.

In one embodiment, sensors could sense biomarkers of the effects andextent of tissue damage at a longer sweat sampling interval than sensorsthat could sense biomarkers of short term stress or trauma on the body,the trauma sensors having locally higher sweat stimulation than thetissue damage sensors. A higher stimulation would result in a highersweat rate, and therefore a faster refilling of any dead volume ormicrofluidic volumes between the skin and sensors, and therefore aneffectively shorter sampling interval. Such stimulation could also occurat regular or irregular intervals, as needed for different biomarkers.

FIGS. 4-6 show different potential configurations of sweat pads, eachsuitable for use in the present invention.

FIG. 4 shows one of an alternate arrangement of a sweat stimulation andcollection pad 66. The source 46 and the adhesive 30 of FIG. 2 isreplaced with a single layer 68, which includes the adhesive, such as ahydrogel based adhesive that contains pilocarpine or other sweatstimulant. This electrode 70 activates the pilocarpine in layer 68,causing sweat generation. The sweat, in turn, flows through microfluidiclayer 72 to a sensor (not shown).

FIG. 5 shows one of an alternate arrangement for a sweat stimulation andcollection pads 76. The sensor 78 is immediately adjacent to the skin 12and therefore eliminated the need for the functionality of amicrofluidic component such as microfluidic element 64. For example, thesensor 78 of FIG. 5 could be fabricated on a plastic film substrate andperforated with holes (not shown) that allow for pilocarpine from source80 to be iontophoretically dosed through the sensor 78 and adhesive 82to the skin 12. In this embodiment, electrode 84, when activated, causessweat generation, which immediately contacts sensor 78. Again, aplurality of three pads could be employed and activated by a commoncircuit. Adhesive 82 may not be required in all applications. Forinstance, the collection pad 76 could be a subcomponent of a largerdevice that is affixed to skin and therefore collection pad 76 is heldin adequate proximity with skin, or a band or strap or other mechanismemployed to hold collection pad 76 against skin.

FIG. 6 shows another alternate arrangement for a sweat stimulation andcollection pad 86. The pad 86 includes a gate 88 and a sensor 90. Thegate 88 is a structure that starts or stops fluid flow and can be awater soluble member which acts as a sweat barrier until sufficientsweat is generated to dissolve the gate to allow fluid flow. Or it canbe a water soluble, water permeable member that initially promotes fluidflow and stops fluid flow after a certain amount of sweat has passed.Therefore the gate 88 can open fluid transport of sweat to the sensor 90only at a time when desired, typically only when sweat stimulation isapplied for that pad and sweat flow is robust enough that the localsweat sample is fresh and representative of a good chronologicalsampling of solutes in sweat. Gate 88 could also be pressure actuated bysweat itself, or activated by means such as electrowetting,thermocapillarity, or any other suitable means. Gate 88 could bereversible, for example, it could open, close, open, and close again.Pad 86 further includes a porous electrode 91, pilocarpine source 93 andadhesive layer 95. This is particularly useful for single-use sensorssuch as those that are easily disrupted by surface fouling with time orthose with such a strong affinity for the biomarker to be detected thatthey are unable to detect later decreases of the biomarkerconcentration. Again, for such single use sensors the gating could be aphysical gating of a microfluidic component carrying the sweat, orsimply the sensors activated as sweat is stimulated in a manner adequateto bring sweat to the sensor. With further reference to FIG. 6 and incombination with other embodiments of the present invention, a devicecould also consist only of pad for sweat stimulation and with gateswhich couple sweat stimulation and sweat collection with one or moremicrofluidic components. For example, one sensor could be fed bymultiple microfluidic components which stimulate sweat and collect it asneeded. The gates could allow flow of fresh/stimulated sweat whileblocking unstimulated sweat. The gates could also not be needed, justallowing sweat to flow freely to the sensor as it is generated by one ormore stimulation pads.

FIG. 7 is a diagram of a portion of components of a device 94, affixedto skin 12 by adhesive 108, similar to device 10 of FIG. 2 arranged in amanner that provides significantly different function and a uniquemanner of separation of stimulation and collection/sensing components.In some cases such as sensing ion concentrations, pilocarpine and/orother solutes or solvents or an electric field used for its delivery orpurposes could alter the readings of the sensors 96, 98. Therefore thestimulation electrodes 100, 102, their respective sweat stimulationssources with pilocarpine 104, 106, and adhesives 108 are located nearbut spaced from sensors 96 and 98, as well as any collection pad, ifused. The pilocarpine stimulation, if performed by iontophoresis,follows electrical field, a pathway indicated by arrows 112. This canresult in stimulation of sweat while not bringing sensors 96 and 98 intoconcentrated contact with pilocarpine or other chemical sweat stimulant,or if desired reducing electric field or current on or near sensors 96and 98. In the example embodiment shown in FIG. 7, the sensor 98 willreceive significant sweat because the stimulation is occurring beneathas caused by electric field and iontophoretic current applied betweenelectrodes 100 and ground electrode 114. Again, each of the sweatstimulation pads are preferably attached to timing circuitry thatpermits selective activation and deactivation of each pad.

FIG. 8 is applicable to any of the devices of FIGS. 2-7 or otherembodiments of the present invention. If stimulation electrode/padcontact to the skin is inadequate, this can be detected as an increasein impedance and that pad can be deactivated for purpose of skin safetyand/or inadequate stimulation. The sweat sensing device 116 affixed toskin 12 by adhesive 117, as shown in FIG. 8, senses impedance of thecontact of the electrode 118 (with pilocarpine source 119 andmicrofluidic component 121) with the skin 12 and/or the contact ofcounter electrode 120 with the skin 12 where ‘contact’ refers to directcontact or indirect contact but which has adequate and/or uniformelectrical conduction with the skin. Inadequate contact can causeinsufficient sweat stimulation, an increase in current density andadditionally therefore cause irritation, damage, burns, or otherundesirable effects with the skin or with the function of the device.Measurement of electrical impedance includes obvious related measuressuch as capacitance, voltage, or current which also give a measure ofimpedance. If the impedance exceeds a preset limit by circuit 122, theelectrode 118 can be deactivated. This reduces the likelihood of burningthe skin. Furthermore, if sweat stimulation pads are redundant (one ormore), the embodiment illustrated in FIG. 7 can allow the presentinvention to select the ‘adequate’ or ‘best’ ones for use with one ormore embodiments of the present invention. In an alternate embodiment ofthe present invention, inadequate stimulation can also be measured byone or more known means of measuring sweat rate, such as impedances,lactate concentration, or sodium or chloride concentration.

In an alternate embodiment, each counter electrode and iontophoresiselectrode of the embodiments of the present invention can be placedclose to each other and/or controlled in conjunction with each other. Toallow prolonged sweat stimulation but to limit areas of skin to shorterterm stimulation, each sweat stimulant source and electrode could beutilized sequentially. For example, if a safe protocol for stimulationwas found to be up to 1 hour, but 24 hours of stimulation and sensing isneeded, then 24 sets of electrodes and sources could be usedsequentially. Also, after a period of time, stimulation can bereactivated under a given electrode and source (for example, sweatgeneration could become ‘tired’ and after ‘resting’ for some time, beenacted again at the same time). Therefore multiple sequences or timingsof stimulations and collections are possible, to enact sampling of sweatat multiple intervals or continuously for a longer period of time thanis conventionally possible. Multiple microfluidic components could beassociated with one-way flow valves as well, reducing fluid flowcontamination or confusion between multiple fluidic pathways orelements. The time scales listed herein are examples only, andstimulation for less regular, more short, or even longer total durationsare possible.

In an alternate embodiment, each stimulation pad, even if with orwithout a microfluidic component, can have a volume between skin andsensor such that reduced stimulation is allowed while still providingadequate chronological resolution (sampling interval). Conventionalsweat stimulation requires >1 nL/min/gland flow of sweat to allow aproper sampling volume. The present invention allows the sweatstimulation to be reduced to <2 nL/min/gland, preferably <0.5nL/min/gland using sweat stimulation concentrations/dosages as found inthe literature (e.g. Buono 1992, J. Derm. Sci. 4, 33-37) appropriate forsuch reduce stimulation and sweat rates. Such an alternate embodimentcan be desirable, because it can reduce one or more of the undesirableaspects or side-effects of sweat stimulation or prolonged sweatstimulation. Enabling calculations for reduced stimulation, sweat rates,volumes and areas, were provided in the background section.

For sensors located on the palms or soles the skin is very thick and ifbecomes wet for prolonged periods of time the sweat can slowunacceptably or stop altogether as skin swells to the point where sweatducts become pinched off. Such state is visibly noticeable as ‘wrinklingof the skin’ after the skin is exposed to water for a longer period oftime. Therefore for prolonged sensing, a dessicant, hydrogel, or otherabsorbent material can be placed over top or adjacent to the sensors ofthe present invention to enable longer term viability of sensing of thepalm or sole with reduced concern of skin swelling/wrinkling and reducedsweat flow rate either natural or stimulated.

With reference to FIGS. 9 and 10, an alternate embodiment of the presentinvention is shown using block diagrams to convey function of a moreadvanced example subset of components of devices of the presentinvention. The components shown for the device 124 in FIG. 9 has apilocarpine source reservoir 126 which contains a sweat stimulatingcompound such as pilocarpine, a fluidic component 128, and a samplingcomponent 130, all of which are integrated in a device resting on skin12. Fluidic component 128 and sampling component 130 could also be oneand the same where sampling component 130 is just an extension of thefluidic component 128. In an example embodiment, an electrode 132 isprovided to reservoir 126, thus enabling iontophoretic dosing ofpilocarpine through fluidic component 128 and into skin 12 as indicatedby arrow 134. This dosing of pilocarpine generates sweat, which isinitially wetted into fluidic component 128 and then transported intosampling component 130 as indicated by arrow 136. The sweat may travelalong a partially separate flow path than the pilocarpine to minimizeinteraction between the sweat and the pilocarpine, but full separationis not required. The above example could be achieved by usingpilocarpine placed into a hydrogel which forms the reservoir 126,stacked onto a thin piece of paper or other fluid porous material forthe fluidic component 128, which is then connected to another piece ofpaper or tube for the sampling component 130. The sampling component130, or even fluidic component 128, may contain or be in fluidic contactone more sensors (not shown), or may simply store sweat for lateranalysis by a sensor external to the device 124. In further examples,the iontophoresis could be continuous, and allow continuous sampling ofnon-charged biomarkers or solutes in sweat, or the iontophoresis couldbe intermittent and between dosing by iontophoresis both charged andnon-charged biomarkers or solutes in sweat could be sampled.

Components 126 and 128 in alternate designs could also be one and thesame, as could also be true for components 128 and 130. To minimizesweat solute diffusion into or out of the reservoir 126, the reservoir126 may be made of a material such as a gel that is slow to diffusion ofsolutes but fast in allowing iontophoretic transport of solutes. Anon-limiting example would be an ion-selective membrane with selectivitypartial to pilocarpine or substances with charge or makeup similar topilocarpine.

FIG. 10 shows a sweat sensing device 138 with similar features as FIG.9, but also includes a membrane 140 and a storage component 142. Thestorage component 142 may simply collect and store sweat as it issampled through the sampling component 146. The storage component 142,could for example, be a hydrogel which swells and increases in volume asit takes up a fluid like sweat. The sampling component 146 can includeone more sensors providing a chronological measure of biomarkerconcentration in sweat instead of a time-integrated measure that wouldoccur if the sensor were instead placed in the storage component 142.Sensors could also be placed at or near the location of fluidiccomponent 144, or at or near the skin 12, as described for previousembodiments of the present invention. The membrane 140 is any componentthat allows pilocarpine or other compound diffusion or iontophoresisthrough the membrane 140, but which reduces or prevents diffusion ofbiomarkers or solutes in sweat through the membrane 140 and back intothe pilocarpine reservoir 148. Furthermore, membrane 140 can serve as abarrier to fluidic contact between reservoir 148 and other components ofthe device 138 of the present invention to increase storage life aspilocarpine gels typically are hydrated and can diffuse out pilocarpineover time into other porous media they are brought into contact with.Reservoir 148 and membrane 140 could also be one and the same, withmembrane having selective transport for sweat stimulating substance. Forexample, selective membranes or materials that are partial to transportsweat stimulant can be known membranes partial to transport of only onetype of ion polarity (for example for favoring the charge of the sweatstimulant ions) or partial transport to molecules as small as but notsubstantially larger than the sweat stimulation molecule through simpleprinciples of size exclusion. Further examples can be found throughliterature on ‘selective molecular sieves’.

As a result, sweat stimulation and sampling can be integrated in thesame device with less interference between the two. For example, themembrane 140 could be a track-etch membrane with 3% porous open area,and the pilocarpine concentration and iontophoretic driving voltageincreased on the reservoir 148 such that the amount of pilocarpine dosedcan be similar or equal in effectiveness to that that of a reservoir 148placed directly against the skin 12. Because the membrane 140 only has3% porous area, diffusion of solutes in sweat into the reservoir 148 isreduced substantially up to 30×. The fluidic component 144 may beadequately thick that any pilocarpine coming through holes or pores inthe membrane 140 would have adequate distance before reaching the skinto spread out into a more even concentration and current density intothe skin. Membrane 140 could be any material, film, ion-selective gel,or other component which transports a sweat stimulating component suchas pilocarpine, but which minimizes the transport of other all orparticular sweat solutes back into the reservoir 148. Membrane 140therefore could also be a fluidic or ionic switch or valve, which isopened during a short period of time for iontophoresis of pilocarpine,but closed once an adequate pilocarpine dose has been released from thereservoir 148. Furthermore, membrane 140 can serve as a barrier tofluidic contact between reservoir 148 and other components of thedevices of the present invention to increase storage life as pilocarpinegels typically are hydrated and can diffuse out pilocarpine over timeinto other porous media they are brought into contact with. For caseswhere the membrane 140 is a fluidic switch an electrode may be providedwith the fluidic component 144 to complete iontophoresis of pilocarpineeven after the fluidic switch 140 is closed to pilocarpine transport.Example fluid switches include those actuated by electrowetting,switchable selective ion channels, and other means achieving the samedesired functionality.

In an alternate embodiment of the present invention, with furtherreference to FIG. 10, reservoir 148 will include an electrode to driveelectrophoresis (not shown), and the electrode may also be utilized tomeasure sweat rate by electrical impedance with skin 12. In one exampleembodiment, to allow proper measurement of sweat rate by impedance, theelectrical impedance of membrane 140 should be similar to or preferablyless than the electrical impedance of skin 12 (these two impedancesbeing in series, such that skin impedance dominates and improves thequality of sweat rate measurement by impedance). Using first principles,this is easily achieved assuming electrical conductivity of fluids inthe device 138 to be roughly equal, and the sum of the electricalresistance of pores in membrane 140 to be less than the sum of theelectrical resistance due to sweat ducts in skin 12. Therefore, membrane140 could be selected such that it has a low enough porosity to helpprevent contamination between reservoir 148 and fluidic component 144,but also having high enough porosity such that it does not block properimpedance measurement of skin 12. Fortunately, impedance can be measuredusing a small signal AC waveform, which results in little or no netmigration of pilocarpine or other charged sweat stimulant.

In an alternate embodiment of the present invention, with furtherreference to FIGS. 9 and 10 the reservoir of pilocarpine and fluidiccomponent can also be switched in locations (trading locations asillustrated and described, but retaining their primary functionalitiesand the advantages/features as described for embodiments of the presentinvention).

For the embodiments of FIGS. 9 and 10, it is desirable for someapplications that the fluid capacity, or volume, of the fluidic andsampling components 144 and 146 be as small as possible. This isimportant, because if the fluidic and sampling components 144 and 146have a large volume, the components will effectively integrate theconcentrations of solutes in sweat over a longer period of time, andlimit the ability to achieve a time-resolved measurement of solutes insweat. Furthermore, any delay on transporting sweat from the skin 12 toa sensor can cause degradation in concentrations of some biomarkers orsolutes in sweat, and therefore minimum volume of the fluidic andsampling components 144 and 146 is also desirable.

An example stack-up of an embodiment of the components comprising thedevice 138 is shown in FIG. 11. As represented by arrow 150 of FIG. 10,sweat will flow along sampling component 146 to storage component 142while pilocarpine flows directly to the skin 12 as shown by arrow 152.

Sweat stimulation can be applied continuously or repeatedly over longperiods of time so long as the currents utilized for iontophoresis andtotal doses are properly controlled. In yet another embodiment of thepresent invention devices can include controllers which allow sweatstimulation for periods of hours to potentially more than a day induration.

In some cases, even with careful electrical controllers and microfluidicdesign, skin irritation could occur, and in these cases in yet anotheralternate embodiment of the present invention includes sweat stimulationpads that are <50 mm² in order to reduce perceived irritation by theuser, even less than 10 mm² or less than 2 mm². These ranges for thepresent invention are much smaller than the commercial Wescor product,which has a stimulation pad that is >1 cm² (>100 mm²), because largeamount of sweat needs to be collected given the highly manual nature ofthe sweat collection and sensing. Assuming ˜100 sweat glands/cm², a 50mm² stimulation pad could collect sweat from on average 50 glands, 10mm² on average 10 glands. If the stimulation pad is placed in regionswhere sweat gland density is >350 glands/cm² then a 2 mm² stimulationpad could cover on average >6 glands and most likely at least one glandalways with careful placement. The present invention may also use muchlarger sweat stimulation pads, if it is acceptable for the applicationand/or other embodiments of the present invention are utilized tosuitably reduce irritation caused by sweat stimulation.

In some cases, even with careful electrical controllers, reducedstimulation area, and advanced microfluidic design, skin irritationcould occur, and in these cases in yet another alternate embodiment ofthe present invention, the pilocarpine reservoir can also contain aniontophoretically transported or diffusing anti-inflammatory, numbingagent, or pain-relieving agent (hydrocortisone, for example, or otheriontophoretically delivered pain relieving agents). This could allowlonger stimulation and usage than otherwise deemed acceptable by theuser. Ideally, the anti-inflammatory or pain relieving/numbing agentdelivered will have properties such as: (1) not interfering with sweatstimulation (not suppressing it); (2) have a similar charge polarity asthe sweat stimulating substance and be co-delivered to the same sitewith it. For example, deliver combinations of stimulant oranti-inflammatory/numbing agents, such as “name (example chargepolarity)”: (1) stimulants—Pilocarpine (+), Acetylcholine (+),Methacholine (+), Phenylephrine Hydrochloride (+), Isoproterenol (+);(2) anti-inflammatories/numbing agents—such as Dexamethasone (−),Hydrocortisone (+ or − depending on compound), Salicylate (−),Lidocaine. Several of such substances or molecules can also be alteredin charge to work with positive or negative polarity. Furthermore, evenoppositely charged substances could be co-delivered to the same locationas sweat extraction takes place, for exampling, using an electrodearrangement using features similar to that shown in FIG. 7 where thenumbing agent would be delivered using electrodes that are side by sidewith electrodes delivering sweat stimulant, and in between suchelectrode pairs sweat would be collected. Furthermore, agents to reducepain or irritation or swelling could be allowed to penetrate bydiffusion over time (charged or uncharged), as some agents such ashydrocortisone work well based on diffusion alone and do not need topenetrate overly deeply into the skin. Numerous such combinations arepossible, the key requirement being delivery either simultaneously or atother times which allow both sweat stimulation and chemical orpharmalogical reduction of irritation, pain, or inflammation. Anexcellent reference, included herein, is Coston and Li, Iontophoresis:Modeling, Methodology, and Evaluation, Cardiovascular Engineering: AnInternational Journal, Vol. 1, No. 3, September 2001 (C ° 2002).

The reservoir may also contain a surfactant or other substance that cancause cell death, cell rupture, or increase skin cell membranepermeability, in order to facilitate biomarker release from the bodyinto the sweat being sampled. The reservoir may also contain solventsknown to increase the effectiveness of iontophoretic delivery.Furthermore, techniques such as electro-osmosis can be used continuouslyor intermittently to promote extraction of biomarkers from the cellssurrounding a sweat duct or from the skin directly. Also, for longduration sweat stimulation, the iontophoresis could potentially causeelectrolysis of water and therefore high concentrations of acids orbases at the two or more electrodes required for iontophoresis.Therefore in yet another alternate embodiment of the present invention,the electrodes contacting components, such as that contacting thereservoir or electrode, may also be equipped with buffering agents, orthe electrodes themselves undergo oxidation or reduction in order tosuppress undesirable side-effects of water electrolysis and/or pHchanges.

With further reference to the example embodiments of the presentinvention, sweat generation rate could also be actively controlled todecrease, by iontophoresis of a drug which reduces sweating, such asanticholingerics including glycopyrrolate, oxybutynin, benztropine,propantheline. For example, a sweat retarding chemical could replacepilocarpine in reservoir 126 of FIG. 9A. Sweat generation rate couldalso be reduced by administering a solvent to the skin such as glycolswhich can swell the top layer of skin and pinch off the sweat ducts suchthat sweat generation rate is reduced by constriction of flow of sweatto the surface of skin. Other antiperspirant compounds or formulations,such as Aluminum chloride are possible as well. Why would one want toslow the sweat generation rate? Two non-limiting examples include thefollowing. Firstly, some sensors or subcomponents could foul or degradein performance more quickly as fresh sweat is brought to them, or thegeneral maximum usage time of the patch decrease as a result of a sweatgeneration rate that is too high. Second, some solutes or properties ofsweat could be read more reliably at lower sweat generation rates, inparticular low concentration solutes could have more time to diffuseinto slowly flowing sweat inside the sweat gland/duct and therefore alower sweat generation rate could produce a higher concentration whichcould be more easily sensed by a sensor. Furthermore, some solutes aregenerated by the sweat gland itself during high levels of sweatgeneration (such as lactate) and could interfere with sensors for othersolutes or sensors trying to sense lactate diffusing into sweat fromblood.

This has been a description of the present invention along with apreferred method of practicing the present invention, however theinvention itself should only be defined by the appended claims.

What is claimed is:
 1. An apparatus to measure or collect sweat over anextended period of time, comprising: a plurality of sweat stimulationpads, wherein each sweat stimulation pad includes: a sweat stimulant, anelectrode, and a sweat stimulant carrying medium, and wherein the pad isadapted to deliver the sweat stimulant to skin to generate sweat; one ormore sweat collectors; one or more sensors for measuring acharacteristic of an analyte in sweat, wherein the sensor is in fluidiccommunication with the sweat collector; and a timing circuit adapted toselectively activate and deactivate said plurality of sweat stimulationpad for one or more limited periods of time, wherein each sweatstimulation pad is selectively operable by said timing circuit.
 2. Theapparatus of claim 1 wherein at least two pads have different sweatstimulation rates.
 3. The apparatus of claim 1 wherein each of said padsincludes a sweat flow path leading to a sensor.
 4. The apparatus ofclaim 3 wherein each sweat flow path leads to the same sensor.
 5. Theapparatus of claim 3 wherein each flow path leads to a separate sensor.6. The apparatus of claim 3 wherein at least one flow path includes agate selectively operable to allow sweat flow only during a selectedtime.
 7. The apparatus of claim 1, further comprising an electricalsensor associated with each sweat stimulation pad, said sensor adaptedto detect direct or indirect contact between said pad and skin, whichpermits deactivation of said pad.
 8. The apparatus of claim 7 whereinsaid electrical sensor detects impedance.
 9. The apparatus of claim 1,further comprising at least one sensor which receives sweat resultingfrom stimulation by said stimulation pad wherein said sensor and saidstimulation pad are not fluidically coupled.
 10. The apparatus of claim1 wherein at least one sweat stimulation pad of the plurality of sweatstimulations pads is configured to stimulate an area of less than 50 mm²to reduce irritation perceived by the user of the apparatus.
 11. Theapparatus of claim 1 wherein at least one sweat stimulation pad of theplurality of sweat stimulations pads is configured to stimulate an areaof less than 10 mm² to reduce irritation perceived by the user of theapparatus.
 12. The apparatus of claim 1 wherein at least one sweatstimulation pad of the plurality of sweat stimulations pads isconfigured to stimulate an area of less than 2 mm² to reduce irritationperceived by the user of the apparatus.
 13. The apparatus of claim 1wherein each of said collection pads includes a gate operable to preventflow of sweat to a sensor.